The present invention relates to a screw fastening machine having a vertical guide which holds a screw when this screw is driven into a member to be fastened.
Unexamined Japanese patent publication No. 11-262871, assigned to the same applicant as that of this application, discloses a vertical guide positioned beneath a driver bit for holding a screw. A holding force of this vertical guide suppresses a lifting of a screw fastening machine caused by a reaction force acting from the screw driven into a material to be fastened.
FIG. 6 shows a vertical guide 41 disclosed in this prior art. The vertical guide 41 is attached to the lower end of a nose portion 36. The vertical guide 41 includes a pair of opposed guide members 22 which are swingable about their pivots. The guide members 22 are resiliently urged by springs 24 so as to close a chuck portion of the vertical guide 41.
Respective guide members 22 have screw holding faces 27 which are configured into pyramid faces directing downward. The screw holding force of guide members 22, especially the force for holding a screw head, substantially suppresses the lifting of this screw fastening machine.
The screw holding force becomes large with increasing inclination of each screw holding face 27.
As the screw head needs to be held until the screw fastening operation is almost finished, the guide members 22 are attached to the lowermost end of the machine body. In other words, the guide members 22 are positioned closest to the member into which the screw is driven.
According to the above-described screw fastening machine, however, when a pressing force applied to the machine body is insufficient, the machine body tends to lift upward due to a reaction force acting from the screw when the screw head is not yet held by the guide members 22. In this case, the reaction force is transmitted to the axes of a piston and a driver bit, i.e., transmitted to the axis of the screw fastening machine. Hence, a handle portion which is held by a user""s hand will receive a significant rotation moment.
The rotation moment acting to the handle portion of a screw fastening machine possibly causes a driven screw to decenter from the axis of the machine body. The screw head may not be evenly held by respective screw holding faces 27 of guide members 22. Such unbalance of holing forces given from the screw holding faces 27 will let the screw come out of the guide members 22. The driver bit may exit out of the engaging grooves on the screw head.
In this manner, according to the conventional screw fastening machine, a duration the guide members 22 hold the screw head is insufficient for surely suppressing the machine body from lifting upward.
In view of the foregoing problems of the prior art, an object of the present invention is to provide a screw fastening machine which is capable of surely preventing the machine body from lifting due to a reaction force acting from a screw driven into a member to be fastened.
In order to accomplish this and other related objects, the present invention provides a screw fastening machine comprising a driver bit driven by an air motor and an air piston so as to reciprocate in an axial direction and rotate about its axis, a screw feeding portion for feeding screws one by one to a predetermined portion beneath the driver bit, a nose portion for guiding a screw when this screw is pushed out by the driver bit, and a vertical guide attached to a front end of the nose portion for holding the screw guided by the nose. The vertical guide of this invention has a chuck portion for holding a screw head, and opening and closing of the chuck portion is regulated depending on a mutual position between a machine body and a member into which the screw is driven.
According to a preferable embodiment of the present invention, it is preferable that a push lever has a stopper portion for restricting the opening motion of the chuck portion. The vertical guide has guide pin portions, and the chuck portion is in a closed condition when the stopper portion is brought into contact with the guide pin portions.
It is also preferable that the chuck portion is constituted by two opposed members which are resiliently urged to contact with each other. At least one of the opposed members has a screw holding face which is inclined with respect to an advancing direction of the screw so that a clearance from the screw holding face to the other opposed member decreases as a position approaches a distal end of the chuck portion.